Checks whether a given named constant exists (PHP 3, PHP 4, PHP 5)
bool defined ( string name )

Returns TRUE if the named constant given by name has been defined, FALSE otherwise.

例 1148. Checking Constants

/* Note the use of quotes, this is important.  This example is checking
* if the string 'CONSTANT' is the name of a constant named CONSTANT */
if (defined('CONSTANT')) {

If you want to see if a variable exists, use isset() as defined() only applies to constants. If you want to see if a function exists, use function_exists().

See also define(), constant(), get_defined_constants(), function_exists(), and the section on Constants.